1. Field of the Invention
The present invention relates to a die and method for driving a rivet and more particularly to such a die which is configured in a particular way relative to the shape of the rivet head to drive the rivet in an improved manner, and to the method performed in the use of the die.
2. Background Art
It is a general practice in the aircraft industry, as well as in other industries, to join metal parts to one another by means of rivets. One common type of rivet is one having a generally cylindrical shank portion that fits into the hole that is formed through the two parts which are to be joined, and a head portion having a diameter moderately larger than that of the shank, with this head fitting against an outer surface of one of the parts to be joined. To set the rivet, a bucking bar or anvil is positioned against the outwardly protruding end of the shank, while a rivet gun is placed against the head of the rivet.
More specifically, the rivet gun is provided with a die that has an impact end that is contoured to fit against the head of the rivet. In operation, the rivet gun has a mass which is mounted in the gun chamber for reciprocating motion. When the gun is operated, this mass impacts or hammers against the die, with the impact forces imposed on the die being transmitted into the rivet. The protruding end portion of the shank is upset (i.e. expanded), and the main portion of the shank that remains in the hole generally expands slightly so as to be in proper engagement with the cylindrical surfaces of the two parts that define the hole or opening in which the rivet is placed.
While the above described operation of fastening metal pieces by rivets is on the surface a relatively simple mechanical operation, there are complexities which lead to more subtle considerations relating to the effectiveness of the connection made by the rivet. With regard to the connecting function of the rivet itself, generally the major forces which are exerted on the rivet are sheer forces (i.e. opposing forces directed perpendicular to the lengthwise axis of the shank of the rivet which would tend to cause one part of the rivet to sheer laterally from the other). Further, for the rivet to be an effective load bearing member, it is necessary for the rivet to be in proper contact with the hole forming surfaces of the two metal parts along substantially the entire length of that hole.
Another important consideration is that the driving of the rivet should be accomplished in a manner so as to minimize any possibility of structural damage to the parts being joined. Since material must be removed to form the holes for the rivets, the structural loads must be carried by the remaining material of the piece surrounding the rivet holes. This makes it essential that this remaining material surrounding the holes not be stressed, scratched or otherwise damaged so as to diminish its load carrying capacity.
The rivet configuration which has been used quite commonly in the aircraft industry for a number of years is one where the head has a lower annular flat surface adapted to engage the outer surface portion of one of the parts that surrounds the rivet hole. The upper surface of the rivet head then curves upwardly and inwardly in a radial direction from the circumference of the lower head surface toward a middle portion of the rivet head. Then the top middle surface portion of the rivet head is nearly flat or only moderately rounded. The die which is in the rivet gun and which contacts the rivet head has at its end a recess which substantially matches the contour of the upper surface of the rivet head. In general, this particular die configuration has been found to work effectively with this type of rivet.
A more recent development was to modify the configuration of the head of the rivet by eliminating a certain amount of rivet material at the outer circumferential portion of the rivet head, and also eliminating a small amount of material from the top middle surface portion of the rivet head. Not only does this produce a small weight saving for each rivet (which weight saving can become significant when one considers the large number of rivets used in an entire airplane structure), but this permitted the rivet holes to be spaced more closely to certain structural components so that there could be a savings in the amount of material used for the structure itself. For example, when a right angle section is being joined to a flat piece, the curved juncture portion where the two flanges of the angle section meet is a critical structural area which should not be damaged in any way. By reducing the diameter of the rivet head, it is possible to move the rivet holes closer to the location of the curved portion of the angle section without causing damage to or interferring in any way with the structural integrity of the curved juncture portion of the angle section.
The present invention is directed particularly to the problems associated in driving a rivet having the modified head configuration as described immediately above. A search of the patent literature has revealed a number of patents relating generally to riveting machines. In general, while the patents resulting from the search deal with various aspects of the riveting process, these patents were not particularly informative relative to the problems toward which the present invention is directed. Accordingly, these patents are listed below without any specific comments. These patents are:
U.S. Pat. No. 1,180,028, Gooding; PA1 U.S. Pat. No. 3,147,647, Downes; PA1 U.S. Pat. No. 3,491,930, Hill; PA1 U.S. Pat. No. 3,557,442, Speller; PA1 U.S. Pat. No. 3,672,553, Doring; PA1 U.S. Pat. No. 3,977,229, Alvi et al; PA1 U.S. Pat. No. 4,060,189, Vargo Jr. et al; PA1 U.S. Pat. No. 4,101,064, Vargo Jr. et al; PA1 West German No. 750,369.
In view of the foregoing, it is an object of the present invention to provide a die for driving a rivet and also a method of driving the rivet, which die and method are particularly adapted to work effectively with the modified type of rivet head described above.